Convective Gas Mixing in the Airways of the Human Lung - Comparison of Laminar and Turbulent Dispersion

A mathematical model of gas transport in the airways of the human lung with numerical solution of the corresponding differential. equation is presented. The model takes into account, along with the summed cross section of. the Weibel lung model, both convection and longitudinal dispersion of helium and sulphur hexafluoride in air. Simulation was performed using two dispersion coefficients corresponding to laminar and disturbed flow. Moreover, since the dispersion coefficients are closely related to the velocity, five constant flow rates were used for each computation and each simulation. Comparison between the model responses to laminar and turbulent dispersion was made in order to determine which plays the preponderant role in gas transport in the human lung. In addition, agreement between the experimental time constant of CO₂ elimination during high-frequency ventilation and the predicted mixing time constant was satisfactory. It is concluded that Taylor laminar dispersion cannot play a significant role in the human airways; however, it seems that convective gas mixing with disturbed dispersion-corresponding to a quasi-steady state - can account for most observed gas transport phenomena during spontaneous breathing and high-frequency ventilation.